1. Field of the Invention
This invention generally relates to an electromagnetic clutch, such as for use in controlling the transmission of power from an automobile engine to a refrigerant compressor in an automobile air conditioning system, and more particularly, to a structure of a friction surface of the electromagnetic clutch.
2. Description of the Prior Art
An embodiment of Japanese Utility Model Application No. 52-151258 is essentially illustrated in FIG. 1. Electromagnetic clutch 10' is intended to be coupled to a refrigerant compressor in an automobile air conditioning system. Compressor housing 11 is provided with a cantilevered tubular extension 12 surrounding an extension of drive shaft 13 of the compressor. Drive shaft 13 is rotatably supported in the compressor housing 11 by bearings (not shown). Axis X is the horizontal axis about which hub 24, armature plate 26, and clutch rotor 15 rotate. All radial dimensions are derived with respect to axis X.
Clutch rotor 15 is rotatably supported on tubular extension 12 through bearing 16 which is mounted on the outer surface of tubular extension 12. Clutch rotor 15 is made of magnetic material, such as steel, and comprises outer annular cylindrical portion 151, inner annular cylindrical portion 152 and axial end plate portion 153, which connects outer and inner cylindrical portions 151 and 152 at the axial forward end (to the right in FIG. 1). Annular U-shaped cavity 17 is defined by portions 151, 152 and 153. A plurality of V-shaped grooves 18 are provided on the outer peripheral surface of outer annular cylindrical portion 151 for receiving belt 40 to couple the compressor to the output of the automobile engine (not shown).
Axial end plate portion 153 includes one or more concentric slits 19 which are disposed on one or more concentric circles. These slits 19 define a plurality of annular or arcuate magnetic pieces with the surface of the poles being on the axial end plate portion 153.
Electromagnetic coil 20 is disposed in annular cavity 17 of clutch rotor 15 to supply magnetic flux 50 for attracting armature plate 26 to axial end plate portion 153 of rotor 15. Coil 20 is contained within annular magnetic housing 21 having a U-shaped cross section. Housing 21 is fixed to supporting plate 22, which is secured to the axial end surface of compressor housing 11 by a plurality of rivets 221. A small air gap is maintained between coil housing 21 and clutch rotor 15.
Hub 24 is disposed on the terminal end of drive shaft 13. Hub 24 is secured to drive shaft 13 by nut 25. The hub 24 comprises tubular member 241 secured on the terminal end of drive shaft 13 and radial flange portion 242 extending radially from the axial end of tubular member 241. Flange portion 242 is integrally formed with tubular member 241. Alternatively, flange portion 242 may be formed separately from the tubular member 241 and fixed on the tubular member 241 by any known securing method, for example, by welding.
Annular armature plate 26 is composed of magnetic material, is concentric with hub 24, and faces the axial end plate portion 153 with a predetermined axial air gap 1 therebetween. Armature plate 26 is elastically connected to flange portion 242 of hub 24 through a plurality of leaf springs 27. Armature plate 26 includes friction surface 26a facing friction surface 153a of axial end plate portion 153 of rotor 15. Stopper plate 28 and one end of each leaf spring 27 are secured by rivets 29 to the outer surface of flange portion 242 through spacing member 30. The other end of each leaf spring 27 is fixed to armature plate 26 by rivet 31 to support armature plate 26 flexibly for axial movement upon deflection of leaf spring 27.
Thus, when electromagnetic coil 20 is energized, armature plate 26 is attracted to axial end plate portion 153 of rotor 15, and thus friction surfaces 26a and 153a engage each other. Drive shaft 13 is then rotated together with rotor 15 by the engine output through leaf spring 27 and hub 24.
When electromagnetic coil 20 is not energized, armature plate 26 is separated from rotor 15 due to the elasticity of leaf springs 27. Rotor 15 is thus rotated by the engine output, but the compressor is not driven.
Referring to FIG. 2, to enhance the torque transmission from the rotor to the armature plate, friction member 60' of non-magnetic material is fixedly disposed within annular groove 26b' formed near the radially-outermost edge of friction surface 26a of armature plate 26. Accordingly, magnetic flux 50 radially penetrates through annular portion 26c' of armature plate 26. Furthermore, the depth of annular groove 26b' is uniform across the distance from the radially-outer edge of the annular groove 26b' to the radially-inner edge of the annular groove 26b'. When viewed from a position horizontally perpendicular too the axial direction, the cross section of annular groove 26b' is rectangular. That is, thickness L1 of annular portion 26c' of armature plate 26 is uniform from the radial outer edge of the annular groove 26b' to the radial inner edge of annular groove 26b'. The width of annular portion 26c' is denoted as r.
In this prior art device, the peripheral area of the radially inner side of annular portion 26c' is smaller than peripheral area of the radially outer side of annular portion 26c' with a difference equal to "2.pi.rL1". Therefore, magnetic resistance at the periphery of the radial-inner side of annular portion 26c' is larger than the magnetic resistance at the periphery of the radial-outer side of annular port 26c'.
Accordingly, the number of magnetic lines of flux which radially penetrate through annular portion 26c' is restricted by the peripheral area of the radially inner side of annular portion 26c', thereby creating the situation where an increase in electric power is not accompanied by a proportionate increase in electromagnetic attraction.
Furthermore, the above defect, if solved by increasing the thickness of the armature plate, is then replaced by another defect, such as an increases in the weight of the electromagnetic clutch.